1. Field of the Invention
The present invention relates to a two-way CATV system for setting a plurality of communication channels in an upstream communication line to a central equipment (hereinafter referred to simply as a center) from a plurality of end terminal equipment (hereinafter referred to simply as terminals) by means of frequency division multiplexing.
2. Description of the Related Art
In conventional two-way CATV systems with communication channels arranged between terminals and a center unit, in order to reduce upstream noises in an upstream communication line, a plurality of switches called bridger gates 810 and 870 and so on adapted for turning on and off in response to instructions from the center unit are disposed in the vicinity of branching points or junctions 850, 840 and so on between a trunk line 830 and branch lines 820, 821 and so on, so that only a bridger gate belonging to a branch line having a terminal transmitting a signal is closed in compliance with an instruction from the center unit, while bridger gates associated with the branch lines having no terminal transmitting a signal are kept open thereby to prevent noises from flowing into the trunk line from any branch line having no terminal which transmits a signal. In FIG. 8, assume that only a terminal 861 is transmitting an upstream signal, while terminal 862 and other terminals are not transmitting any upstream signal. A center unit 800 controls the bridger gates 810, 870 and so on, and closes the bridger gate 810 alone, while keeping the other bridger gates including 870 open. Then, only a signal of the branch line 820 associated with the terminal 861 is transmitted to the trunk line, while the signals of the other branch lines 821 and so on are prevented from entering the trunk line, thereby eliminating upstream noises.
This conventional system for eliminating the upstream noises operates to eliminate such noises effectively in the case where there is only one concurrent communication channel in the upstream communication line and therefore only one bridger gate is open at the same time. Nevertheless, there are many cases in which an upstream communication line is used in frequency division multiplexing with a plurality of communication channels set at a time.
In such a case, if communication channels are set at the same time between terminals associated with different branch lines and a center unit (for example, between a terminal 861 associated with a branch line 820 and a center unit 800, and between a terminal 862 associated with a branch line 821 and the center unit 800 in FIG. 8, independently and at the same time), a plurality of bridger gates would be closed at the same time (in the cited example, the bridger gates 810 and 870 are closed at the same time), with a result that noises from a plurality of branch lines would flow into the trunk line, thereby making it impossible to eliminate noises completely.
This problem is aggravated with an increase in the number of frequency multiplexing carrier waves and an increase in the number of communication channels set at the same time.